Influenza is a viral infection that attacks your respiratory system — your nose, throat and lungs. Influenza is caused by three types of RNA viruses called influenza types A, B and C (considered different genera), which all belong to the family Orthomyxoviridae. The disease, colloquially called “flu” in humans, is generally caused by the viruses A and B, which are transmitted by aerosols from infected individuals or via close contact with infected animals.
Influenza hemagglutinin is a surface glycoprotein that binds to sialic acid residues on respiratory epithelial cell surface glycoproteins. This interaction is necessary for the initiation of infection. After viral replication, progeny virions are also bound to the host cell. Neuraminidase cleaves these links and liberates the new virions; it also counteracts hemagglutinin-mediated self-aggregation entrapment in respiratory secretions.
Types of influenza
Influenza type A viruses are known to infect people, birds, pigs, horses, whales, seals and other animals, but wild birds represent the natural hosts for these viruses. The enveloped influenza A virions contain three membrane proteins (HA, NA, M2), a matrix protein (M1) just below the lipid bilayer, a ribonucleoprotein core (consisting of 8 viral RNA segments and three proteins – PA, PB1, PB2), as well as the NEP protein.
Influenza type A viruses can be further divided into subtypes based on two membrane proteins on the surface of the virus. These proteins are called hemagglutinin (HA) and neuraminidase (NA). There are 18 different HA and 11 different NA subtypes (HA1 through HA18 and NA1 through NA11, respectively).
Influenza B viruses are responsible the same spectrum of disease as influenza A; however, influenza B viruses do not cause pandemics. Such property may be a consequence of the limited host range of the virus (only humans and seals), which limits the occurrence of new strains by reassortment. In addition, they are not divided into subtypes, although can be broken down into lineages and strains. Currently circulating influenza B viruses belong to one of the two lineages: B/Victoria and B/Yamagata.
Influenza B virions contain four envelope proteins: HA, NA, NB, and BM2. The BM2 protein is a proton channel that is essential for the uncoating process (akin to the M2 protein of influenza A virus). The NB protein is thought to be an ion channel, not required for viral replication in cell culture.
Influenza C viruses are different in comparison to influenza A and B. The enveloped virions have hexagonal structures on the surface and form stretched cordlike structures (approximately 500 microns in length) as they bud from the cell. Analogous to the influenza A and B viruses, the core of influenza C viruses is composed of a ribonucleoprotein made up of viral RNA and four proteins.
The M1 protein lies just beneath the membrane, similar to influenza A and B virions. A minor viral envelope protein is CM2, which has a function of an ion channel. This virus does not contain separate HA and NA glycoproteins, yet their function is consolidated in one glycoprotein called HEF (hemagglutinin-esterase-fusion). Therefore the influenza virion contains 7 RNA segments, and not 8.
Influenza type C infections cause a mild respiratory illness (comparable to other common respiratory viruses) and are not thought to cause epidemics.
Probably people started to get influenza around the time they started keeping chickens and living in cities – in China and India, about 3000 BC, in Greece about 500 BC, and in North America not until about 1500 AD. But the first good description of influenza comes from Hippocrates, in Greece, who described an influenza epidemic that happened in 412 BC.
The term “influenza” was coined by an Italian in the mid-1700’s to conote a disease resulting from miasma (bad air). The human disease is thought to have arisen about 6000 years ago. A human influenza virus was not isolated until 1933. Wilson Smith, Christopher Andrews, and Patrick Laidrow first identified the virus that causes human influenza only because they found a suitable host for propagation. Laidrow used ferrets in his studies on canine distemper virus, and was able to propagate the flu virus in that same host, as well. Influenza virus was later shown to also infect adult mice and chicken embryos.
The 1918 pandemic also known as the Spanish flu was caused by an H1N1 virus. The 1957 flu pandemic also called the Asian flu was caused by an H2N2 virus. The 1968 pandemic also called the Hong Kong flu was caused by an H3N2 virus. Finally, the 2009 pandemic called swine flu was caused by a novel H1N1 virus.
Global influenza surveillance indicates that influenza viruses are isolated every month from humans somewhere in the world. In temperate regions, influenza activity peaks during the winter months. In the Northern Hemisphere, influenza outbreaks and epidemics typically occur between November and March, whereas in the Southern Hemisphere, influenza activity occurs between April and September. In tropical regions, influenza can occur throughout the year. Although the epidemiology of influenza has been studied for many years, certain features—such as its seasonality, the precise mechanism for the emergence of new variants, and the factors that influence the spread of the disease—are not well understood.
How Flu Spreads
- Most experts believe that flu viruses spread mainly by droplets made when people with flu cough, sneeze or talk. These droplets can land in the mouths or noses of people who are nearby.
- A person might also get flu by touching a surface or object that has flu virus on it and then touching their own mouth, eyes or possibly their nose.
- You may be able to pass on the flu to someone else before you know you are sick, as well as while you are sick.
- Most healthy adults may be able to infect others beginning 1 day before symptoms develop and up to 5 to 7 days after becoming sick.
- Some people, especially young children and people with weakened immune systems, might be able to infect others for an even longer time.
Flu risks and complications
In the majority of cases, flu is not serious – it is just unpleasant. For some people, however, there can be severe complications. This is more likely in very young children, in the elderly, and for individuals with other longstanding illness that can undermine their immune system.
The risk of experiencing severe flu complications is higher for certain people:
- Adults over 65
- Babies or young children
- Pregnant women
- Individuals with heart or cardiovascular disease
- Those with chest problems, such as asthma or bronchitis
- Individuals with kidney disease
- People with diabetes
- People taking steroids
- Individuals undergoing treatment for cancer
- Those with longstanding diseases that reduce immune system function
Some of the complications caused by influenza may include bacterial pneumonia, dehydration, and worsening of chronic medical conditions, such as congestive heart failure, asthma, or diabetes. Children may develop sinus problems and ear infections.
The symptoms of influenza (flu) appear suddenly and often include:
- Fever of 100.4°F (38°C) to 104°F (40°C), which can reach 106°F (41°C)
- Body aches and muscle pain (often severe), commonly in the back, arms, or legs.
- Infants with the flu also may seem fussy all of a sudden or just “not look right.”
- Pain when you move your eyes.
- Fatigue, a general feeling of sickness (malaise).
- Loss of appetite.
- Ear pain
- Sore throat
- Runny nose
- A dry cough
- Severe vomiting
- Nasal congestion
Several types of influenza tests are available. The method used often depends on availability and reason for testing.
Rapid flu tests—depending on the method, a flu test may be completed in a healthcare practitioner’s office or near a hospital patient’s bedside in 20 minutes or less, or the sample may be sent to a laboratory, with the results available the same day
Rapid influenza diagnostic test antigen detection – these tests detect viral antigens in nasal secretions. One main disadvantage of the rapid influenza antigen test is the high rate of false-negative results. Rapid antigen tests generally detect 50-70% of influenza cases.
Real Time Reverse Transcription Polymerase Chain Reaction (RT-PCR) and other molecular tests – these tests detect viral genetic material (RNA) in respiratory samples. They are generally more sensitive and specific for the influenza virus than rapid antigen detection tests. They can more accurately detect the virus when it is present and rule it out when it is not. Depending on the test used, they will identify 66% to 100% of influenza cases.
Viral culture—in this test, the virus is actually grown and further identified in the laboratory as influenza A or B and the strain present, or as another respiratory virus. The availability of viral cultures is decreasing as most laboratories adopt molecular assays to detect viral infections. Viral cultures are costly and more difficult to perform and take up to 3 to 10 days to provide a result, which makes them less useful for determining whether or not someone has the flu and for making treatment decisions.
Additional laboratory tests may be used in conjunction with influenza testing to help rule out other types of infections with similar symptoms and/or if the cause of the infection is unclear. Examples include:
- RSV test—to detect respiratory syncytial virus, a virus that often infects young children and the elderly
- Strep test—to check for group A streptococcus, the bacteria that cause strep throat
Treatments for flu
As flu is caused by a virus, antibiotics cannot help, unless the flu has led to another illness caused by bacteria. Antivirals, such as oseltamivir (Tamiflu) and zanamivir (Relenza), may be prescribed in some circumstances.
Painkillers can alleviate some of the symptoms, such as headache and body pains.
Some painkillers, such as aspirin, should not be given to children under 12.
Individuals with flu should:
- Stay at home
- Avoid contact with other people where possible
- Keep warm and rest
- Consume plenty of liquids
- Avoid alcohol
- Stop smoking
- Eat if possible
It is a good idea for people that live alone to tell a relative, friend, or neighbor that they have flu and make sure someone can check in on them.
There are two types of vaccinations, the flu shot and the nasal-spray flu vaccine. The flu shot is administered with a needle, usually in the arm – it is approved for anyone older than 6 months, including healthy people and those with chronic medical conditions.
- The nasal-spray flu vaccine is a vaccine made with live, weakened flu viruses that do not cause illness.
- Seasonal flu shot. A flu vaccine will contain three influenza viruses:
- Influenza (H3N2) virus
- Influenza (H1N2) virus
- One B virus
Serious side effects usually begin within a few minutes to a few hours after receiving the shot. Possible serious side effects of vaccination include:
- Difficulty breathing
- Swelling around the eyes or lips
- Racing heart
- Behavior changes
- High fever
You can reduce the risk of infection by getting vaccinated and practicing good hand and respiratory hygiene to protect yourself and others:
- Stay home when you are sick
- Wash your hands regularly with soap and water or use an alcohol-based hand rub
- Wash your hands before touching your eyes, nose, and mouth
- Use a tissue, or the inside of your arm, when you cough and sneeze
- Throw tissues away immediately and wash hands
- Don’t share items such as cigarettes, cups, lipstick, toys, or anything which has come into contact with the mouth or nose
- Stay at least 1 metre away from people who have flu-like symptoms
- Clean frequently touched surfaces regularly, such as door handles, taps, tables, benches, and fridge doors (flu viruses can be removed using household detergent)